1. Field of the Invention
The present invention relates to a polyester film excellent in weather resistance, having a good planar surface state and having a low thermal shrinkage, to its production method, to a backsheet for solar cells and to a solar cell module.
2. Description of the Related Art
A solar cell module generally has a structure of (sealant)/solar cell element/sealant/backsheet (hereinafter referred to as BS) as laminated in that order on glass through which incident sunlight passes. Concretely, the solar cell element is generally encapsulated with a resin (sealant) such as EVA (ethylene-vinyl acetate copolymer) or the like, to which BS is stuck. As the backsheet for the solar cell, heretofore used is a polyester film.
However, in general, when a polyester film is used as the backsheet (BS) for solar cells for a long period of time, it may readily peel from the solar cell, and when it is left in weathered environments such as outdoors for a long period of time, BS may readily peel from the sealant such as EVA.
Accordingly, BS is required to have high weather resistance. As such BS, there is known a weather-resistant polyester film of which the weather resistance has been improved (for example, see Patent Reference 1, JP-A 2008-130642). The weather-resistant polyester film has a relatively high intrinsic viscosity (IV) and has a relatively low terminal carboxyl group (terminal COOH) content (terminal COOH content: AV), and therefore has high hydrolysis resistance.
As BS having improved weather resistance, Patent Reference 1 proposes a laminate BS produced by laminating a film which is prepared by forming an easy-adhesion layer on the polyethylene terephthalate (PET) substrate surface of a gas barrier film, or that is, a transparent alumina-deposited film, followed by heat treating it to make it have a thermal shrinkage of 1.0%, a white PET and a weather-resistant PET of Toray's X10S.
In addition to peeling by weathering, when the solar cell module is left in an environment in which it is exposed to high temperatures for a long period of time (for example, on the roof in summer or in desert) and in case where it has a high thermal shrinkage, BS may peel from the sealant therein. Accordingly, BS is required not to peel from the other members of the solar cell module owing to the difference in the shrinkage between them, in addition to having excellent weather resistance.
In regard to this point, mere investigation of the thermal shrinkage of ordinary PET in a PET prepared by forming a water vapor barrier layer on an ordinary PET and given weather resistance is made in Patent Reference 1; however, nothing is investigated therein relating to the thermal shrinkage of a film prepared by imparting weather resistance to a PET film itself, as in the present invention, and in addition, nothing is disclosed therein relating to the thermal shrinkage of a bas barrier film, especially to a detailed thermal shrinkage controlling method in heat treatment in the coating and drying step to produce the film.
Some references disclose a method of modifying a weather-resistant polyester film to have a lowered thermal shrinkage. Concretely, Patent Reference 2 (JP-A 2003-211538) describes production of a polyester film having a thermal shrinkage of from 3 to 20%, and IV of 0.65 dL/g or so, by stretching a polyester film, then thermally fixing it at 190° C. and thermally relaxing it in the cross direction of the film in a tenter.
Patent Reference 3 (JP-A 2007-150084) describes production of a polyester film having a thermal shrinkage of from 0.1 to 1.0%, AV≦15 eq/ton, and IV≧0.8 dL/g, by stretching a polyester film, then thermally fixing it at 235° C., thermally relaxing it in the cross direction of the film in a tenter, and thereafter processing it for low-temperature thermal treatment at a low tension.
Patent Reference 4 (WO2007/040039) describes production of a polyester film having a thermal shrinkage of at most 2% and IV of from 0.6 to 1.2 dL/g, by stretching a polyester film, then thermally fixing it at 220° C., thermally relaxing it in the cross direction of the film in a tenter, and thereafter processing it for low-temperature thermal treatment at a low tension.
Patent Reference 5 (JP-A 8-187777) describes a method comprising solid-phase polymerization of a polyester resin having IV of from 0.5 to 0.6 dL/g to make it have IV of from 0.70 to 0.75 dL/g and thereafter melt-casting it to form a film. The patent reference further describes production of a polyester film having AV≦25 eq/ton by stretching the formed polyester film and then relaxing the tension thereof along with high-temperature heat treatment at 240° C. (from Tm −80 to Tm −20° C.).
On the other hand, with the recent popularization of solar cell modules, stable operation thereof has become desired in various installation sites and in various installation methods. Accordingly, BS of solar cell modules is desired not to have surface planarity defects such as curling or waving and is desired to have excellent surface planarity.
Given the situation, the present inventors investigated the methods described in Patent References 1 to 5, and have known that, though the thermal shrinkage of the polyester films described in these patent references could be improved in some degree owing to the treatment for thermal treatment reduction, but the weather resistance and the surface planarity of the films are still unsatisfactory.
For example, in the methods described in Patent References 2 to 5 where the thermal fixation temperature after stretching is lowered to reduce the thermal shrinkage of the formed films, it may be predicted that, owing to the advanced molecular alignment (tension) inside the film, water could hardly penetrate between the molecules and therefore the hydrolyzability of the film may lower and the weather resistance thereof could be thereby enhanced. In the films obtained according to the method of lowering the thermal fixation temperature after stretching, the molecular alignment may be broken when heated, and therefore the thermal shrinkage of the films may increase. To solve this problem, it may be predicted that the methods in Patent References 2 to 4 comprise thermal relaxation treatment. However, the present inventors' investigation has revealed that the thermal relaxation treatment merely reduces and controls the molecular alignment inside the film, but the weather resistance and the thermal shrinkage are in a trade-off relation therebetween, and therefore it is impossible to attain both weather resistance enhancement and thermal shrinkage reduction on a high level. In the methods described in Patent References 2 to 5, the draw ratio is increased in some degree for enhancing the weather resistance of the formed films, thereby providing high-orientation polyester films. Accordingly, the stretched film has a high thermal shrinkage, and is therefore greatly shrunk during thermal relaxation treatment. As a result, it has been known that the obtained films deform like corrugated sheets and their surface planarity is poor.
On the other hand, Patent Reference 1 uses ordinary PET. PET itself has poor weather resistance and, in addition, as a result of the present inventors' investigation, it has been known that reduction in the thermal shrinkage of PET is unsatisfactory and the surface planarity thereof is poor.
Accordingly, BS heretofore known in the art is in fact unsatisfactory in that it could not satisfy both the improvement in the surface planarity and the weather resistance thereof and the reduction in the thermal shrinkage thereof.